Cell: Essay on Cells in Human Body

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The body of any living organism is made up of cells. Cells are very minute in size and extremely complicated in structure. Human is no exception. Each cell is basically a unit of protoplasm, which is said to be the “material basis of life”. The protoplasm is the combination of cytoplasm and nucleus.

The cytoplasm is semi-fluid, jelly-like, hyaline substance; its outer surface is much more thick to form the boundary of the cell. This specialized boundary of the cell is known as cell membrane or plasma membrane. The cell membrane controls the in-and-out of various chemical substances. Various living and non-living bodies are found in the cytoplasm.

Of the living bodies the mitochondria and the Golgi body are very important. The mitochondria are the minute, semi-solid body enclosed in a membrane with a complex internal structure. Although the main constituents of mitochondria are protein and fat, it also contains several enzymes, notably oxidative enzyme systems. The Golgi body is the local clump of material present in the cytoplasm.

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Its functions are not clearly known, but seem to be associated with the formation of secretions. The vacuole is important as the fluid-filled space within the cytoplasm of a cell, which controls the cell sap isotonic with cytoplasm by expelling or entering the water from the environment, following the process of osmosis. The centrosome is the region of differentiated cytoplasm containing centriole. The centriole is the minute granule present in many resting cells, just outside the nuclear membrane.

The nucleus is denser than the cytoplasm and it is regarded as the dynamic centre of life. Its shape is absolutely spherical. A membrane, known as nuclear membrane surrounds it to keep the core protected. The nuclear cavity remains filled up with a dense jelly-like substance, known as nucleoplasm. Some delicate thread-like structures that suspend in the nucleoplasm are called nuclear reticulum or chromatin network.

The threads are easily stainable by basic dyes; they are made up of a substance, called chromatin. Apart from these, one or two small spherical bodies are found in the nucleoplasm, known as nucleolus. The nucleus exerts a direct influence on different activities of the cell and plays a great role in transmitting hereditary characters from the parents to offspring.

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The cell can be divided into two types – the somatic cell or body cell and the germ cell or reproductive cell. The somatic cell helps in construction and maintenance of different bodily structures. It can also be sub-divided into nerve cells, muscle cells, etc. in accordance with their nature and function.

The germ cell is useful in reproducing new species. It can be sub-divided into sperms or male sex cells and ova or female sex cells. The zygote is also a cell, which is formed by the fusion of an egg or ovum, and a sperm. The human ovum is spherical in shape, and about 1/7th of a millimeter or about 1/175th of an inch in diameter.

Though it is the largest of all cells in human body, still not visible in naked eye. In premature stage, a sperm cell is similar to a somatic cell consisting of a nucleus and a mass of cytoplasm. But a mature sperm cell is long and thread-like with an enlarged head formed by the nucleus. It also possesses a long slender tail and a conical middle piece. The tail is used in movement.

A new life starts when a sperm fertilizes an ovum. After the union of the sperm and the ovum, the fertilization takes place when the head and the middle piece of the sperm sink into the egg. The tail of the sperm is left outside. However, the head of the sperm starts functioning as a normal nucleus by absorbing fluid from the cytoplasm of the ovum.

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Fertilization is thus a process of the nuclear fusion. The sperm loses its identity and the fertilized ovum or egg is called a zygote, which begins to divide into two, four, eight and so on, by the process of cell division. From the early stage, some of these cells are set apart to form the germ cells while others go to form various body parts, known as somatic cells.

The cell division is not a simple process; complicated changes have been noted in the substances of the nucleus. In this regard it is necessary to know about the chromosomes. The chromosomes are the slender, rope-like bodies that usually occur in pairs and found in the nucleus. The number of chromosomes remains constant in each species, which generally ranges between 2 to 200.

The size and form of chromosomes also vary from species to species. In fact, differences of chromosome constitution mark off one species from another.In case of man, the usual number of chromosomes is 46. There are altogether 23 pairs of chromosome; 22 pairs act as autosomes and the other pair is known as sex-determining chromosome or sex chromosome.

Chromosomes are composed of two kinds of nucleic acids and two main types of proteins. Chromatin is the most important nucleo-protein in chromosomes. The main nucleic acid of the chromosome is Deoxyribonucleic acid or DNA. In 1951, a renowned biologist James Watson with his chemist friend Francis Crick proposed the structure of DNA.

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The other kind of nucleic acid in the chromosomes is the Ribonucleic acid or RNA, which exists in cytoplasm. DNA samples from different tissues of the same species are all identical in composition. RNA not only differs from DNA; the amount of ribonucleic acid in the chromosomes varies from tissue to tissue.

It is found in large quantities in the cytoplasmic particles of the cell. In some cases, the nucleolus also contains large quantities of RNA. It is believed by the scholars that RNA plays an essential role in protein synthesis and carries instruction to the cytoplasm of a cell. In general, the DNA content of an egg is greater than that of a sperm. A variety of suggestions have been made regarding the relationship between the DNA and protein components of the chromosomes but no steady conclusion has yet been achieved.

As a matter of fact the nucleus and the cytoplasm exist in a state of symbiosis. The nucleus depends on the cytoplasm for its energy supply and also for the supply of materials out of which new nucleic acid molecules and chromosomal proteins can be synthesized. On the other hand, the cytoplasm is fully dependent on the nucleus for the maintenance of its essential biosynthetic mechanisms.